This invention relates to positive locking devices for use with electrical motors, especially permanent magnet direct current motors used with aircraft accessories. Another aspect of the invention relates to circuitry for switching a permanent magnet motor and providing dynamic braking thereto. In particular, the invention relates to three-input, solid-state, switching circuitry for switching a permanent magnet motor and providing dynamic braking thereto. A third aspect of the invention relates to circuitry for providing dynamic braking for a permanent magnet motor having dual armatures. "Split series" direct current ("DC") motors have commonly been used in order to power aircraft accessories. Split series DC motors have two stator coils which are wound in opposite directions. The two coils allow the direction of rotation of the motor to be chosen simply by energizing one or the other of the oppositely wound coils.
A simple three-wire connection has commonly been used to select the direction of rotation of a split series motor. Such a three-wire connection comprises a power return connection which is coupled to the return lead of each coil, a power input connection for the first coil, and a power input connection for the second coil. The direction of rotation of the motor is chosen by applying power to the power input of either the first or the second coil. Split series motors previously have been popular because this simple three-wire connection can be used to control the motor.
Split series motors, however, suffer from certain disadvantages. One disadvantage is that employing two coils leads to relatively larger, heavier, and more costly motors as compared to permanent magnet ("PM") DC motors. Another disadvantage is that split series motors, in contrast to PM motors, cannot be dynamically braked.
As the name suggests, PM motors use permanent magnets, rather than wound coils, to provide a magnetic field in which the armature rotates. In order to reverse the direction of rotation of a PM motor, the polarity of the electrical connections to the armature brushes must be reversed.
In order to control a PM motor using a three-wire connection which is analogous to the split series switching arrangement, solid-state switching circuitry having MOSFETs has been used. This switching circuitry allows a user to use a three-wire connection in which one connection is used for the power return, and power is applied to one of the other two connections in order to select the direction of rotation. Such solid-state switching circuitry provides the advantage that it is transparent to a user accustomed to using a split series motor.
However, the solid-state switching circuitry for PM motors described in the preceding paragraph does not take advantage of the fact that PM motors can be dynamically braked. When a motor is turned off, the armature of the motor usually continues to spin due to its rotational inertia. The spinning armature of a PM motor generates a back electromotive force ("EMF"). Dynamic braking is provided by switching the armature into a short circuit, or a load resistor, when the motor is turned off, thereby causing the back EMF to generate a current. The energy dissipated by the current causes the spinning armature to quickly slow down.
In order to provide a simple three-input control of a PM motor and quickly slow down the motor after power has been removed, it would be advantageous to provide a three-input, solid-state switching circuit for controlling a PM motor which also provides dynamic braking to the motor.
After an electric motor has come to rest, various forces may cause the motor to rotate. Frictional brakes have been used in order to hold a motor in a fixed position after it has been shut off. However, frictional brakes have the disadvantage that the braking force is proportional to the spring force used to apply the braking, and that undue wear may occur on components of the motor due to frictional forces.
In view of the shortcomings of frictional brakes, it would be advantageous to provide a positive locking device to prevent rotation of the armature of an electric motor while the motor is not energized. In a positive locking device the motor is mechanically held fixed, rather than frictionally held fixed.